Instruments are needed with improved accuracy and with fewer of the limitations found in earlier I-V data acquisition systems for solar cell arrays. The general goal of performing measurements on any piece of equipment is to determine how well the equipment is presently functioning, and to further determine the ability of the equipment to perform a specific task. Whether the testing is done in the design, test, installation, or field service stage of the life of the equipment, the accuracy of test data is of utmost concern. For solar cell arrays, the need is to obtain as accurate a reading of each panel as possible at every point in its I-V characteristic load curve. It is thus important to devise a very accurate test instrument. If lower accuracy is demanded, the readings may be rounded off, but once the measuring instrument is built, the user is forced to live within its accuracy limits, so if higher accuracy is demanded, the instrument cannot be used. Therefore, an object of the present invention is to provide an instrument for measuring I-V characteristic data of such high accuracy as to be able to satisfy the demand of any requirement.
To obtain an accurate measurement of a specified variable, the values of all other variables which affect the reading must be set to known levels. Instruments used in the past for solar panel I-V characteristic measurements have done this by setting the current to be drawn from the panel under test and measuring the panel output voltage, or by setting the voltage across the panel and measuring the current output. Either method is very effective for a substantial portion of the I-V characteristic load curve. However as the knee of the curve is approached (where voltage begins to change at a much greater rate as current is changed or vice versa), the accuracy of the readings decrease. This is due to the relatively high change in voltage or current caused by a small change in current or voltage in this area of the load curve. The maximum number of readings in this area is also restricted by the limit of minimum current or voltage regulation steps that can be controlled.
In addition to a need for accuracy, there is a need for the instrument to be portable so that it may be carried into the field where a solar array has been installed, often in rugged terrain where it is used to recharge storage batteries for continuous power to systems at remote locations. Digital data storage in the field with portable instrumentation is at best a very difficult task. Under the rigors of various climates and atmospheres encountered in the field testing of photovoltaic solar arrays from Alaska to Panama, in the Mojave Desert, and on mountain tops in California and elsewhere, magnetic data storage media (tape or disc) will not stand up because magnetic tape and discs have been known to lose magnetic material under such extreme conditions. In the desert, sand particles and other artifacts abrade the media surfaces. Humidity and heat, or cold, create unreliable storage surfaces in the tape or disc media. Consequently, there is a need for a reliable means to collect and store digital data in the field for subsequent use. The storage medium should be readily insertable and removable from the instrument, and transferrable to a system for data display and/or a digital data processor for evaluation and analysis, such as a solid state digital storage device.
Given a reliable record media for the storage of I-V characteristic measurements, there is a need for selection of the data format to use for optimum use of the limited storage space in the media. One of the prime considerations in selecting a data format is the amount of storage available. If a high volume storage device, such as a cassette tape recorder is used, data may be stored in an easily transferable format such as ASCII encoded BCD data. Since a cassette tape would not be a reliable record media for this instrument, a low volume solid state storage device must be used. It therefore follows that the data must be packed. The object of data packing is to get as much data into a small space as possible without losing accuracy.